Four cards. Which ones would you turn over?

Why the beer round felt easy

The three rounds are the same puzzle. Each rule says "if this, then that". Only two cards can break such a rule: the one that shows "this", and the one that shows "not that". The card showing "that" can never break it, whatever is on the back. Yet it is the card most people reach for, because it looks like the rule.

Peter Wason built the letters version in 1966. Only about one person in ten gets it right. In 1982 Richard Griggs and James Cox gave students the beer version: 73% got it right. The same students went 0 for the letters. The logic was the same. What changed was that everyone knows how to catch someone drinking under age, and almost nobody knows how to catch a rule about vowels.

The same game in a trading room

"When my indicator flashes BUY, the stock rises." The tempting check is to look at the weeks the stock rose and see if the signal was there. That is the "Rose" card. It can never prove the rule wrong. The cards that can are the BUY weeks, to see if any fell, and the weeks that fell, to see if any had a BUY. A chart book full of winning signals is a pile of "Rose" cards.

Sources

1. Wason, "Reasoning", in Foss (ed.), New Horizons in Psychology (Penguin, 1966), 135–151.
2. Wason, "Reasoning about a rule", Quarterly Journal of Experimental Psychology 20 (1968), 273–281.
3. Griggs and Cox, "The elusive thematic-materials effect in Wason's selection task", British Journal of Psychology 73 (1982), 407–420.